With realization of higher performance and higher quality of liquid crystal displays, as polarizing plate protective films used for polarizing plates, there are being sought polarizing plate protective films exhibiting long-term durability of polarizers under high temperature and high humidity conditions, and exhibiting performance to endure severe ambient variation against dimensional stability and curling properties of the polarizing plates.
Long-term resistance to heat due to backlighting is required specifically for a polarizing plate adjacent to the backlight. Recently, to increase light efficiency of the backlight, a liquid crystal display has been designed so that a luminance enhancing film is sandwiched between the backlight and the liquid crystal cell. However, light, having passed though this luminance enhancing film, is frequently polarized. Accordingly, with regard to the polarizing plate protective film, a film has been demanded to be designed so that no birefringence is expressed by heat resulting from the backlight.
Currently, a film employing a cellulose ester as a material is commonly used for a polarizing plate protective film of a liquid crystal display. A solution casting film forming method is commonly used to produce a cellulose ester film from the viewpoint of ensuring flatness. Such a film tends to expand in the film thickness direction as a solvent evaporates, whereby the refractive index in the thickness direction is smaller than that of the film in-plane direction. In addition thereto, as a result of taking into account the display mode of a liquid crystal display and retardation of another retardation film or member, in order to ensure a specific viewing angle of the liquid crystal display, there have been sought a film, as an optimal polarizer protective film, featuring the same refractive index both in the thickness and the in-plane directions, and further a polarizing plate protective film featuring a relatively high refractive index in the in-plane direction.
Patent Document 1 discloses that via addition of an ethylenic polymer, there is obtained a film exhibiting minimal deterioration of a polarizer under high temperature and high humidity conditions, as well as exhibiting minimal retardation in the thickness direction. However, it became clear that the level of durability, having been currently required, of the polarizer under high temperature and high humidity conditions could not be realized only by adding an ethylenic polymer. In addition, it became clear that as more ethylenic polymer was introduced in order to reduce retardation, the polarizer tended to be deteriorated more severely.
Patent Document 1: Unexamined Japanese Patent Application Publication No. (hereinafter referred to as JP-A) 2003-12859